1. Technical Field
This invention relates to polymer electrolyte membranes having superior proton conductivity, oxidation resistance, hot water resistance, and fuel impermeability. The electrolyte membranes are suitable for use in solid polymer fuel cells and are produced by first graft polymerizing acrylic acid derivatives or vinylketone derivatives as monomers onto polymer substrate films and by then performing selective conversion to a sulfonic acid group of hydrogen atoms on the carbon atom adjacent to the carbonyl in the ketone or carboxyl group on the graft chains. The present invention also relates to a process for producing such polymer electrolyte membranes.
2. Background Art
Solid polymer fuel cells have high energy density and hence hold promise for use as power supplies to household cogeneration systems, mobile communication devices and electric vehicles or as simplified auxiliary power sources. Such fuel cells require polymer electrolyte membranes that are long-lived and have high durability.
In solid polymer fuel cells, the electrolyte membrane not only acts as a proton conducting “electrolyte” but also has the role of a diaphragm that prevents the fuel hydrogen or methanol from directly mixing with oxygen. This electrolyte membrane must satisfy the following requirements: high enough chemical stability to withstand prolonged large current, in particular, high resistance in acidic aqueous solutions (acid resistance), high resistance against peroxide radicals (oxidation resistance), and high heat resistance in the presence of water (hot water resistance); and low electrical resistance. The membrane which also has the role of a diaphragm must satisfy other requirements including high mechanical strength and good dimensional stability, as well as low gas permeability to the fuel hydrogen gas or methanol and to oxygen gas.
Early solid polymer fuel cells employed hydrocarbon-based polymer electrolyte membranes as produced by copolymerizing styrene with divinylbenzene. However, those electrolyte membranes did not have high practical feasibility since they were very low in durability on account of poor acid and oxidation resistance; hence, they were later replaced by Nafion® and other fluorine-containing polymer electrolyte membranes. Nafion® is the fluorinated sulfonic acid polymer membrane developed by Du Pont.
The conventional fluorine-containing electrolyte membranes including Nafion® have superior chemical stability; on the other hand, their ion-exchange capacity is small, only about 0.9 meq/g, and on account of insufficient water retention, the electrolyte membrane dries and its proton conductivity is lowered, or in the case where methanol is used as fuel, the membrane will swell in alcohols or “cross-over” of methanol will deteriorate the fuel cell characteristics.
If, with a view to increasing the ion-exchange capacity, an attempt is made to introduce more sulfonic acid groups, the strength of the membrane decreases so markedly that it may break easily. Therefore, with the conventional fluorine-containing polymer electrolyte membranes, it has been necessary to reduce the amount of sulfonic acid groups to such levels that the membrane strength is retained and the only products that could be obtained had no greater ion-exchange capacity than about 0.9 meq/g.
Another problem with Nafion® and other conventional fluorine-containing polymer electrolyte membranes is that monomer synthesis is so complicated that the price of the product membrane is prohibitive and this has been a great obstacle to the effort in commercializing the solid polymer fuel cell membrane. Under the circumstances, efforts have been made to develop low-cost, yet high-performance electrolyte membranes that can be substituted for Nafion® and other conventional fluorine-containing polymer electrolyte membranes.
An attempt has been made to fabricate an electrolyte membrane for use in solid polymer fuel cells by introducing a styrene monomer into an ethylene-tetrafluoroethylene copolymer (hereinafter abbreviated as ETFE) having a hydrocarbon structure by means of a radiation-induced graft reaction and then sulfonating the introduced styrene monomer (see JP 9-102322 A). However, it has been pointed out that this approach has the disadvantage that during cell operation at elevated temperature in the presence of water, the thermal elimination due to the low hot water resistance of the sulfone groups introduced into the polystyrene or the oxidative decomposition of graft chains causes deterioration that is accompanied by a decrease in the ion-exchange capacity of the membrane (see JP 11-111310 A).
With a view to suppressing the elimination of sulfonic acid groups, an attempt has been made to introduce them not by direct coupling to the benzene ring in an aromatic hydrocarbon such as styrene but by coupling with an intervening alkylene group and it has been reported that this technique is effective to some extent (see JP 2003-100317 A). Thus, introducing sulfonic acid groups not directly into the benzene ring is held effective in improving hot water resistance and oxidation resistance.